In a full-duplex communication system, one communication device, for example, a modem, is transmitting a signal and receiving a signal at the same time. One result is that an echo signal leaks from the transmission path to the receiving path. An echo canceller (EC) is employed to effectively remove or cancel this echo signal from the received signal.
FIG. 1 illustrates a block diagram of a prior art ADSL communication system including two modems A and B that are connected through a physical media (a twisted pair line, for example). Modem A is the Customer Premises Equipment (CPE) side modem and includes Echo Canceller A which performs echo cancellation in the digital domain. Modem B is the Central Office (CO) side modem. This discussion focuses on CPE modem A although the problems discusses are equally applicable to the CO modem B. The transmitting path shown includes Encoder A for encoding data into symbols which is communicatively coupled to modulator A for modulating the symbols into a time-domain digital transmission signal. Modulator A is also communicatively coupled to Echo Canceller A to which it also forwards the time-domain digital transmission signal as a reference signal. Digital to Analog Converter (DAC) A converts the digital transmission signal to an analog time-domain transmission signal which Line Driver A amplifies and sends to Hybrid A. Hybrid A separates the transmitting (TX) and receiving (RX) paths of modem A. Ideally, Hybrid A blocks the TX signal from echoing back into the receiving (RX) path of modem A. A practical hybrid, however, provides only about 20 dB of attenuation from the TX path to the RX path. The receiving path includes Analog Amplifier A for amplifying a received signal. The received signal includes the far-end signal plus any local echo signal (RX+Echo) which leaked through Hybrid A. Analog to Digital Converter (ADC) A converts the amplified received signal to a digital signal which Demodulator A sends to the Echo Canceller A for removal of the local echo signal. Demodulator A receives the modified received signal from Echo Canceller A from which it demodulates symbols which are then decoded into data by Decoder A. One example of an echo cancellation scheme for an ADSL system has been proposed by Minnie Ho, John M. Cioffi and John A. C. Bingham, “Discrete Multitone Echo Cancellation”, IEEE Transactions on Communications, vol. 44, no. 7, pp. 817–825, July 1996, which is hereby incorporated by reference. Utilizing the characteristics of Discrete Multi-tone (DMT) modulation, this digital scheme partially cancels an echo signal in the time domain and cancels the remaining echo signal in the frequency domain.
One problem that can arise is an ADC dynamic range saturation problem wherein the echo signal enters the ADC block before cancellation where it can potentially saturate the ADC and reduce its dynamic range for the desired RX signal. For example, a strong local echo signal entering the ADC with an attenuated far-end received signal significantly reduces the gain available for the desired RX signal from the CO. In a frequency division duplex (FDD) communication system, where the TX and RX bands are separate, this ADC dynamic range saturation problem can be overcome using separation filters. In an overlapped-spectrum system, however, separation filters cannot be used so that an echo canceller (EC) is typically used. The ADC saturation problem in a digital EC has been addressed in the U.S. Pat. No. 6,618,480 by Michael O. Polley and William J. Bright of Texas Instruments. However, the proposed solution is to partially remove the echo in analog and remove the remaining echo in digital and does not provide any method to train its proposed analog/digital ECs.
FIG. 2 illustrates an example of an overlapped and non-overlapped spectrum in a full-duplex communication system, an ADSL system. The upstream (US) signal and the downstream (DS) signal are transmitted within the bands of [f0 Hz, f1 Hz] and [f2 Hz, f3 Hz], respectively. In one example, the FDD system, the downstream spectrum 204 does not overlap the upstream spectrum. In another example, to increase the DS capacity, the DS signal frequency band can be extended to the US band as shown in FIG. 2 so that the downstream spectrum 202 overlaps the upstream spectrum.
When f2<f1, an echo canceller is required at both the CO and CPE sides because DS and US share the band of [max(f1,f2) Hz to f1 Hz]. For illustrative purposes, the discussion focuses on echo cancellation at the CPE side, in which the sampling rate at the receiving (RX) path is X times the sampling rate at the transmission (TX) path, where X is an integer number. In one embodiment, example 1, the signal sampling rate at CPE-RX path is 2208 kHz. The sampling rate at CPE-TX path is 552 kHz. Then X=4 for this embodiment. In another example, example 2, the signal sampling rate at CPE-RX path is 8.832 MHz (corresponding to f3 as high as 3.75 MHz or up to 4.416 MHz) and the sampling rate at CPE-TX path is 1104 KHz (corresponding to f1 as high as 552 KHz). In this case X=8. As illustrated by these examples in the overlapped ADSL system, the transmitted upstream signal sampling rate at the CPE side is significantly lower than that of the received downstream signal sampling rate.
It is desirable to provide an echo cancellation system in which the echo signal is cancelled before analog to digital conversion to avoid dynamic range saturation. It is also desirable to provide an echo cancellation system which can accommodate a system when operating with an overlapped-spectrum and when operating with a non-overlapped spectrum. This is particularly desirable for an ADSL system because in an ADSL system, for example, due to spectral compatibility issues, overlapped-spectrum cannot be used beyond a certain loop length and FDD must be used. It is also desirable that an echo cancellation system eliminates the need for separation filters in an FDD system used for echo suppression. It is also desirable that an echo cancellation system accommodates asymmetric signal sampling rates. It is also desirable that an echo cancellation system can selectively cancel the echo in either the analog domain or the digital domain depending on the applications or modes of operations.